A culture medium in which an organism capable of being cultivated, such as a microorganism or cultured cell, is cultivated can be divided into two broad categories: rich mediums and synthetic mediums. Efficiency of proliferation of the microorganisms and of production of useful substances is often greatly influenced by the composition of the culture medium, which is deemed to be critically important. It is commonly known that a culture medium in which microorganisms are cultivated is configured by a carbon source, a nitrogen source, minerals, vitamins, a specified growth factor, and the like. A rich medium is believed to be ideal for proliferation, whereas a synthetic medium where categories and concentrations of individual chemical substances are known is believed to be incapable of surpassing a rich medium in proliferation.
Conventionally, a YPD culture medium has been widely used as a rich medium used to cultivate yeast, the YPD culture medium being configured by 1% yeast extract, 2% peptone (partial fragment of casein), and 2% dextrose (equivalent to glucose). However, the YPD culture medium is expensive. Therefore, a majority of the usage of the YPD culture medium in typical corporations is between a research phase and a pilot plant phase. However, chemical components configuring the YPD culture medium are not known in specific detail because the YPD culture medium includes natural extracts in its composition, and there is variation in components between production lots. As a result, sizable variation in organic activity such as proliferation and fermentation may occur between lots. During large-scale fermentation production, an inexpensive molasses is almost always used; however, in recent years an upward trend in costs for molasses has been observed, and an inexpensive culture medium to replace molasses has been sought.
Meanwhile, a YNB culture medium (Yeast Nitrogen Base) manufactured in 1946 by Wickerham can be obtained at low cost as a synthetic medium used to cultivate yeast, and is therefore still widely used in research investigating nutritional requirements of mutant strains or the like. However, proliferation speed when cultivating yeast is slow as compared to a rich medium, and therefore the YNB culture medium is not utilized in large-scale fermentation production or the like. In addition, although the structural components thereof are precisely known, what role the structural components play in proliferation and fermentation is unclear. In addition, an MM culture medium, for example, in which 0.5% (NH4)2SO4 and 2% glucose are added to 0.67% YNB culture medium without amino acids and without ammonium sulfate is used as a synthetic medium for experimentation.
Other examples of a yeast culture medium may include an agar-agar culture medium for separation and identification of a high ethanol productivity soy sauce fermentative yeast strain which includes between 3.5 and 10% (w/v) glucose and between 0.8 and 1.2% (w/v) manganese chloride, and does not include 1% (w/v) or more of table salt, in a yeast nutrient culture medium (see, for example, Patent Literature 1); a culture medium for inducing expression of a foreign gene product in a yeast cell, the culture medium including a casamino acid having a concentration of between 20 and 160 g/L (see, for example, Patent Literature 2); a 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)furanone producing culture medium composition in which pentose (as a carbon source) and amino acid (as a nitrogen source) are at least combined (see, for example, Patent Literature 3); a culture medium which is a differential or selective medium for Zygosaccharomyces bailii and Zygosaccharomyces bisporus yeasts, the culture medium including a basic inorganic culture medium supplemented by, for example, vitamins such as myo-inositol 4.0% (w/v), trace elements, glucose and formic acid as sole carbon and energy sources, an adapted acid-base indicator, and (as desired) an antibiotic bacteriostatic agent and agar-agar (see, for example, Patent Literature 4); a lactic acid producing culture medium producing lactic acid by fermentation which cultivates microorganisms including yeast having lactic acid synthesizing abilities, the culture medium including between 10 μg/L and 70 μg/L of folic acid or folic acid precursor (see, for example, Patent Literature 5); a yeast culture medium in a liquid, the culture medium being configured by oleic acid, lactic acid, palmitic acid, glutamic acid, citric acid, potassium ion, calcium ion, zinc ion, iron (ii) ion, manganese ion, biotin, thiamine, pyridoxine, inositol, nicotinamide, folic acid, riboflavin, and choline ion, and not including yeast extract (see, for example, Patent Literature 6); and a method of cultivating eukaryotic cells, including yeast cells and plant cells, using a dry powder cultivation culture medium in which pH is automatically adjusted (see, for example, Patent Literature 7).
Examples of a completely synthetic medium for microorganism or cell proliferation may include a completely synthetic medium for mammalian fibroblasts in which polyvinylpyrrolidone, ascorbic acid phosphate, lipids, and cholesterol have been added to a basic culture medium containing human recombinant epithelial cell growth factor or the like (see, for example, Patent Literature 8); and a completely synthetic non-blood serum medium for cultivating human frozen primary hepatocytes after melting, the culture medium containing glucocorticoid, prolactin, cholera toxin, and hepatocyte growth factor (see, for example, Patent Literature 9). Another example is a method of producing a useful chemical compound that includes various steps of fermenting microorganism strains on an industrial scale in a chemically precise fermentation culture medium configured by essentially chemically precise structural components; and collecting the useful chemical compound from the fermentation culture solution, the chemically precise structural components of the chemically precise culture medium being a carbon source selected from a group configured by carbohydrates such as glucose, lactose, fructose, sucrose, maltodextrin, starch, and inulin; glycerin; vegetable oil; hydrocarbons; alcohols such as methanol and ethanol; and organic acids such as acetate and a higher-quality alkanoic acid, and a nitrogen source selected from a group configured by urea; ammonia; nitrate; ammonium salts such as ammonium sulfate, ammonium phosphate, and ammonium nitrate; and amino acids such as glutamate and lysine (see, for example, Patent Literature 10).
With respect to yeast proliferation, additional examples may include a method of promoting yeast proliferation in which a lipid and protein complex is added to a culture medium (see, for example, Patent Literature 11); a culture medium containing a fungal proliferation promotion composition that contains β-glucan as an active ingredient (see, for example, Patent Literature 12); a microorganism proliferation promotion composition containing a glucose supply source and apiose, and including a fungus such as yeast that contains one part or more of apiose per 100 parts glucose (see, for example, Patent Literature 13); and a yeast culture medium in which a flavonoid identified as a factor promoting proliferation of yeast in molasses is chemically synthesized and added, the culture medium not using molasses-derived components intended to reinforce high sugar dough fermentation activity (see, for example, Patent Literature 14).